[PATCH] Fix typo in pgstat_replslot.c

Hi,

I noticed this could be corrected to `exists` (instead of `exits`) here, same as commit
63ed3bc7f9a66035e0b741aaa542de35a90fe1cc.
```c
    /* Check if the slot exits with the given name. */
    slot = SearchNamedReplicationSlot(name, false);

    if (!slot)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("replication slot \"%s\" does not exist",
                        name)));
```

--
Regards,
Man Zeng
www.openhalo.org